An image forming apparatus, such as a multi-function peripheral or a laser printer, includes a fixing device for fixing a toner image on a sheet. By transmitting heat from a heater to the sheet through, for example, a fixing belt, the fixing device fixes a toner image onto the sheet.
In the fixing device, when an image forming process is repeatedly performed, the temperature of components of the fixing device, such as the fixing belt, can be increased. Heat is removed from the fixing belt mainly by transfer of heat to sheets passing through the fixing device. Therefore, the temperature of the sheet passing area of the fixing belt which is in contact with the sheet is lowered, but the temperature of a non-sheet passing area (an area of the fixing belt which is not in contact with the sheet) remains high. When the temperature of the non-sheet passing area remains high, printing/fixing can be performed sufficiently merely by supplying power to the heater corresponding to the amount of heat transmitted from the sheet passing area to the sheet. Therefore, a technique for controlling the temperature of the heater in the image forming apparatus on the basis of the temperature of the sheet passing area and the temperature of the non-sheet passing area has been proposed.
On the other hand, when the image forming apparatus is stopped for a long time or when the interval between printings is lengthy, the temperature of the various components of the fixing device decreases. In this lowered temperature state, the heat of the heater must also flow to the surrounding components, such as the non-sheet passing area of the fixing belt, in addition to the sheet passing area. In order to perform printing in such a case, it is necessary to determine the heating requirement of the fixing belt by taking into consideration the amount of heat moving from the sheet passing area to the surrounding components.